Shark teeth
Shark teeth are relics of shark evolution and biology. Shark skeletons are composed entirely of cartilage. Often the only parts of the shark to survive as are teeth. Fossil shark teeth have been dated back hundreds of millions of years. The most ancient types of sharks date back to 450 million years ago, and they are mostly known from their fossilized teeth. The earliest known fossil shark teeth come from rock beds in Spain. These are teeth from the shark Leonodus, dating from some 400 million years ago. It is somewhat rare to find fossil shark teeth from this period; therefore, we know relatively little from the early stage of chondrichthyan evolution between about 450 and 380 million years ago. The most common fossil shark teeth, however, are from the Cenozoic Era (65 million years ago). The earliest records of fossilized shark teeth that belong to a living family are from representatives of the Hexanchidae (cow sharks) from about 208–187 million years ago. Overview Sharks and rays have a polyphyodont dentition; that is, they shed old teeth and replace them with new ones throughout their lives. A shark can have hundreds of teeth in its jaw. Sharks, as well as other Chondrichthyes, have the ability to replace their teeth if they become damaged during feeding or fall out due to natural causes. Many ichthyologists have suggested that sharks can lose tens of thousands of teeth within the span of a few years. Many teeth are lost in the feeding process, however, many others are simply shed due to the "conveyer-belt" process. There is one reason why shark teeth are so common in the fossil record, and that is because teeth that have been shed during life may be broken or have worn crowns, but the roots will not be always be fully developed. In contrast, the teeth lost as a result of the death of the creature, will contain all tooth growth stages from simple enameloid caps through intermediate and mature stages of root and crown formation. An example of one such developmental sequence is evident in the associated dentition of the late Albian shark, Paraisurus compressus. Often, collectors assume that a tooth with a poorly formed root is broken, when in reality it might be an incompletely developed non-functional, replacement tooth. The teeth of sharks are not attached to the jaw, but embedded in the flesh, and in many species are constantly replaced throughout the shark's life. When they lose a working tooth it will be replaced by the next tooth behind it. All sharks have multiple rows of teeth along the edges of their upper and lower jaws. New teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. Typically a shark have two to three working rows of teeth with 20 to 30 teeth in each row, e.g. a whale shark have about 300 teeth in each row. The replacement rate has not been measured in most sharks but normally the teeth seem to be replaced every two weeks. The lemon shark replaces its teeth every 8–10 days, and the Great white replaces its teeth about every 100 days for young sharks and about every 230 days for old shark. Most sharks shed individual teeth, but e.g. the Cookiecutter shark sheds the whole lower jaw at once. The lower teeth are primarily used for holding prey, while the upper ones are used for cutting into it. The teeth range from thin, needle-like teeth for gripping to large, flat teeth adapted for crushing shellfish. Fossil shark teeth ''Megalodon'' teeth Carcharocles megalodon teeth are among the most sought after types of shark teeth in the world. These teeth are in extremely high demand by collectors and private investors, and they can fetch steep prices. This shark lived during the Miocene and Pliocene eras, roughly about 16 to 1.5 million years ago. Its teeth on average range between 1.5 to 6.5 inch in length. The largest examples can reach a length of more than 7 inches. These huge teeth indicate that the Megalodon could grow up to more than 16 m (52.5 ft) long, growing bigger than the largest fish alive in the world today, the whale shark. Large numbers of Megalodon teeth have been discovered across both coasts of the United States. The most plentiful locations within the U.S. are the Carolinas, Georgia, and parts of Florida and Virginia. These large sharks disappeared relatively close to the rise of modern man, however, there have been many unconfirmed reports of large carnivorous fish resembling Megalodon sharks in recent times. Megalodon teeth have been discovered that some may argue date as recently as 10,000 to 15,000 years ago. This claim is based on the discovery of two teeth by the HMS Challenger scientific expedition. These teeth were dated by estimating the amount of time it took for manganese to accumulate on them, although it is quite possible the teeth were fossilized before being encrusted. Transitional Teeth Identifying shark teeth is tedious work, especially with all of the damaged, worn, and different teeth. It is even more difficult because of the so-called "In-between teeth". These are teeth that are from a shark species that was evolving into another, different species. An example of this can be seen with Carcharocles agustidens teeth that were in the state of evolving into Carcharocles auriculatus. These are teeth that are no longer auriculatus, but not yet angustidens either. Great White/Mako transitional teeth The most common and most referred to transitional shark teeth are the ones coming from what is believed by some to be an unusual form of great white shark. Great white shark transitional teeth are often characterized for their wide crowns. These teeth can also be identified by the way the serrations fade, being more pronounced near the root, and disappearing close to the tip of the tooth. Many paleontologists now believe that these transitional teeth represent the evolutionary path between Isurus hastalis and the Great White shark. The evolutionary history of the great white shark and its relation to Megalodon are hotly debated. Because of their transitional state, these teeth are rare. These teeth are prized by collectors, hobbyists, and museums. Collecting shark teeth Field collecting Locations include river bed banks, sand pits, and beaches. The most common and simplest way is to walk along the site and collect. Many people prefer to use a shovel and sieve. Many collectors use fork-like garden tools to excavate sand. Many sites provide hard, solid teeth, that are only washed and dried. These teeth are typically worn, because they were frequently moved and redeposited in different areas repeatedly before settling down. Other locations, however, yield perfect teeth that were hardly moved during the ages. these teeth are typically fragile, and great care should be taken while excavating them. Buying teeth There is a small, but established shark tooth trade, and it is up to the collector if he/she wants to 'complete' their collection by buying a few fossils. Many high quality fossils can be found online, and many are often affordable. As well as on the internet, shark teeth may be procured from Rock and Mineral shows, such as the annual Tucson Gem & Mineral Show, and other, smaller, rock shops. The prices of shark teeth depend upon the overall quality of the tooth. There is no definite price for a shark tooth, as the market is based on supply-and-demand. Another way of obtaining shark teeth is to "swap" finds with others at rock shops of mineral shows/conventions. Identification The most often method of shark tooth identification, is to procure a book on sharks teeth and attempt to identify it yourself. Many expert collectors tend to identify their teeth this way, yet others prefer to seek experts willing to help them identify their teeth. Many identified shark teeth are transitional species. Because sharks lose tens of thousands of teeth in their lifetime, their teeth vary greatly, because they are constantly evolving new changes in their feeding style, and their teeth. Pathological teeth If a functional tooth is broken, chipped, or cracked, or damaged while feeding, the tooth cannot be repaired. In fact, broken or damaged teeth are commonly seen in the dentitions of living sharks and rays. Pathological teeth are developmental abnormalities that may be caused by a variety of reasons: genetic mutation, damaged or immature teeth. Pathological teeth develop with distorted or disfigured crowns, and collectors usually have little or no trouble recognizing them. However, not all pathological teeth are easy to identify, and several species have been described on the basis of abnormal teeth. References * Castro, Jose (1983). The Sharks of North American Waters. College Station: Texas A&M University Press. ISBN 0-89096-143-3. * Hennemann RM. Sharks & Rays: Elasmobranch Guide of the World. IKAN-Unterwasserarchiv, Frankfurt, Germany 2001:266-269. * Stevens, John D. (1987). Sharks. New York: NY Facts on File Publications. ISBN 0-8160-1800-6. Category:Sharks